The DNA proviruses of the Snyder-Theilen (ST), Gardner-Arnstein (GA) and McDonough (M) strains of feline sarcoma virus (FeSV) as well as that of feline leukemia virus (FeLV, subgroup B) were molecularly cloned in bacteriophage vectors. Each sarcoma virus is a genetic recombinant containing portions of the FeLV genome linked to transforming gene sequences (onc) transduced from normal cat cellular DNA. The viral onc elements of ST- and GA-FeSV (v-fes) share homologous sequences representing similar portions (exons) of a segmented cat cellular gene (c-fes). The v-fes sequences specify a tyrosine-specific protein kinase (PK) activity. Transformation-defective and nontumorigenic mutants of ST-FeSV encode products lacking PK activity showing that PK is required for morphological transformation. The amino acid sequence of the v-fes coded kinase has been deduced by nucleotide sequence analysis. The transforming gene sequences of M-FeSV represent a distinct onc element showing that FeLV can transduce at least two cellular transforming genes. Both feline onc genes are evolutionarily conserved and are homologous to sequences in the cellular DNA of other avian and mammalian species, including man.